Barium magnesium tantalate (Ba(Mg_1/3Ta_2/3)O₃, BMT) ceramics has excellent microwave dielectric properties. It's the key point to synthesize BMT nano-powders with well-crystallized, uniform grain, good dispersion for perfect microwave dielectric ceramics. BMT nano-powders were prepared by hydrothermal method using tantalum oxide, magnesium nitrate, and barium nitrate as raw materials, KOH solution and isopropyl alcohol as solvents. The crystal structure, morphology, and particle size of the product were characterized by X-ray diffractometer and scanning electron microscope. The effects of KOH concentration, reaction temperature, and reaction time on BMT powders were investigated. The effect of calcination temperature on BMT powders was studied by differential thermal analysis. The results show that the optimum conditions for the synthesis of BMT nano-powders are KOH concentration 2 mol/L, reaction temperature 200 ℃, reaction time 24 h and calcination temperature 1 000 ℃ after freezing-drying. Under this condition, the obtained BMT nano-powders particles are spherically dispersed, and the particle diameter is about 100 nm. This method can be used as a reference for hydrothermal synthesis of BMT nano-powders.